In general terms, the main research conducted in this proposal is directed toward a better understanding of the action of narcotic analgesics and the development of tolerance and physical dependence to these drugs. More specifically, the proposal concerns the following aspects: 1) Characterization of central narcotic receptors. 2) Physiological role of endogenous opioid peptides. 3) Factors involved in the enhancement of narcotic antagonistic potency. 4) Compounds which accelerate the development of tolerance.